In a mobile communication the received signal quality deteriorates extremely due to fading. There exists a diversity technique as an effective method for the fading. The diversity technique is to prevent received signal power from falling on a receiver side. However, there are various restrictions to achieve the diversity in a communication terminal apparatus such as a mobile station. Therefore, in order to achieve on a transmitter side of a base station the diversity that is originally expected to be achieved on a receiver side of the mobile station, transmission diversity has been examined.
In the transmission diversity, as illustrated in FIG. 1, base station 1 transmits signals of the same phase from antennas 1 and 2 to mobile station 2, and mobile station 2 selects a received signal with a higher level transmitted from one of the antennas.
Meanwhile, in the DS-CDMA system, the standardization of the transmission diversity is promoted currently that a base station uses closed-loop transmission diversity. There are three modes in the closed-loop transmission diversity. For example, when mode 2 of the closed-loop transmission diversity is applied, a base station provides a signal of antenna 2 with phase rotation (90° shift) with respect to a signal of antenna 1 to transmit. Using signals transmitted from antennas 1 and 2, the mobile station judges a phase difference to be provided between the both signals, and transmits the phase difference information to the base station. The base station transmits signals according to the phase difference information. This processing is executed for each slot. Therefore, the mobile station receives signals with a phase rotated greatly for each slot.
With reference to FIGS. 2 to 8, phases of received signals will be described below that are received at a mobile station when a base station applies mode 2 of the closed-loop transmission diversity.
The base station transmits common pilot channel signals (common known signals) of the same phase from antennas 1 and 2. At this point, the common pilot channel signal transmitted from antenna 1 is spread with a different spreading code from a spreading code that is used in the common pilot channel signal transmitted from antenna 2.
In transmitting communication channel signals from the base station, since the phase rotation control is not performed at a general condition that is not of the closed-loop transmission diversity, the base station transmits the signals to the mobile station using only antenna 1. In the closed-loop transmission diversity, as illustrated in FIG. 6, a signal to be transmitted from antenna 2 is provided with a phase designated by feedback information transmitted from the mobile station, and thereby the phase rotation control is performed to transmit signals.
When the mobile station receives the signals thus transmitted from the base station, since with respect to common pilot channel signals, different signals are transmitted from antennas 1 and 2, channel estimation is capable of being performed for each transmission antenna. In other words, as illustrated in FIG. 3, the mobile station receives signals with different phase rotations respectively from antennas 1 and 2, and therefore channels are estimated separately on the common pilot channel signal transmitted from antenna 1 and the common pilot channel signal transmitted from antenna 2.
Based on the two channel estimation values, the mobile station determines a phase difference to be provided between signals to be respectively transmitted from antennas 1 and 2, and then notifies the base station of the phase difference (feedback information). The setting of the feedback information is herein explained.
As described above, the base station transmits the common pilot channel signals respectively using antennas 1 and 2. The mobile station performs the channel estimation on the common pilot channel signals, and thereby is capable of calculating a phase rotation amount and amplitude variation due to fading on each of antennas 1 and 2.
As illustrated in FIG. 2, when the base station transmits common pilot channel signals with the same amplitude and same phase (phase=0) respectively from antennas 1 and 2, the mobile station receives signals as illustrated in FIG. 3. In FIG. 3, a is indicative of the phase rotation due to fading provided on the transmission signal from antenna 1, while 3 is indicative of the phase rotation due to fading provided on the transmission signal from antenna 2.
As illustrated in FIG. 4, when the base station transmits communication channel signals with the same amplitude and same phase (phase=0) respectively from antennas 1 and 2, the mobile station receives signals as illustrated in FIG. 5. In FIG. 5, A is indicative of an amplitude variation due to fading provided on the transmission signal from antenna 1, while B is indicative of an amplitude variation due to fading provided on the transmission signal from antenna 2. In the mobile station, signals transmitted from antennas 1 and 2 are combined and received as a signal indicated by a bold arrow as illustrated in FIG. 5. At this point, the phase of the combined vector is Φ before.
In this case, since β−α is about 90°, it is expected that rotating the phase of a signal from antenna 2 by 90° increases the combined vector composed of the signals transmitted antennas 1 and 2. Therefore, the phase of a signal from antenna 2 is set to −90°, and the mobile station notifies the base station of feedback information (phase difference) for requesting the base station to transmit the signal with the phase difference.
When the feedback information is notified to the base station correctly, a communication channel signal is transmitted on a next slot as illustrated in FIG. 6. In other words, the signal with a phase shifted −90° is transmitted from antenna 2. As a result, the mobile station receives signals as illustrated in FIG. 7. At this point, the phase of the combined vector is rafter. Accordingly, even when the fading environment does not change, in the mobile station a phase shift is caused like Φ rafter and Φ before due to the phase addition in antennas on a transmitting side.
On the communication channel of the mobile station, by increasing channel estimation accuracy, control is performed that channel estimated results of a plurality of slots are weighted to be added. This control is performed on the assumption that a phase rotation amount due to a fading variation is small with respect to the number of slots whose channel estimated results are added. However, as described above, when the closed-loop transmission diversity is applied, since the channel estimation value varies even when the fading does not varies, it is not possible to calculate an accurate channel estimation value by averaging channel estimation values of a plurality of slots to use, and thereby reception performance deteriorates.